The present invention relates to an exposure method in lithography process for manufacturing a semiconductor device, and a photomask for the same. More particularly, the present invention relates to a double exposure method for manufacturing a semiconductor device, and a photomask for the same.
Generally, semiconductor devices such as Dynamic Random Access Memory (DRAM) comprise a number of fine patterns, which are formed using a photolithography process. To form the fine patterns through photolithography process, a photoresist layer is first deposited on an object layer. The patterned photoresist layer is subjected to an exposure step to selectively change a solubility of portions of the photoresist layer. Then, a developing process is performed to remove portions of the photoresist layer depending on the solubility of the photoresist upon exposure, thereby forming a photoresist layer pattern through which some portion of the object layer is exposed. Thereafter, the exposed portion of the object layer is removed by an etching process using the photoresist layer pattern as an etching mask The photoresist layer is then stripped off, forming an object layer pattern.
Resolution and depth of focus (DOF) are two defining characteristics of a photolithography process. The resolution R can be obtained by the following equation;R=K1·λ/NA  (1)
where κ1 is a constant determined by composition and thickness of the photoresist layer, λ is wavelength of light used, and NA indicates the numerical aperture of an illuminating system.
Referring to Equation 1, the resolution R can be increased by decreasing the wavelength of light λ, and/or by increasing the numerical aperture NA, thereby forming a finer pattern. However, there are limitations in such an approach when degree of device integration is rapidly increased. Accordingly, to overcome the limitations as described above, various resolution enhancement techniques (RETs) for enhancing resolution and depth of focus have been proposed. For example, optical proximity correction, phase shift mask, off-axis illuminating system, and the like have been used as one or more of the RETs. A double exposure technique has also been used to form very fine patterns on a wafer.
FIG. 1 is a layout view illustrating an example of a pattern suitable for a conventional double exposure method. Referring to FIG. 1, a wafer 100 comprises a cell region 110 in which dense patterns 130 are formed, and a peripheral circuit region 120 in which an isolated pattern 140 is formed. Alternatively, the dense patterns 130 can be formed in the peripheral circuit region 120 instead of the cell region 110. Although the isolated pattern 140 is shown as a line or a stripe in FIG. 1, it can have other shapes. In addition, the patterns formed on the cell region 110 are disposed in one direction, whereas the pattern formed on the peripheral circuit region 120 can be disposed in more than one different directions.
FIGS. 2 to 5 are simplified diagrams illustrating a conventional double exposure method for forming the pattern in FIG. 1.
Referring to FIGS. 2 and 3, a primary exposure is performed using a first photomask 200 and a first illuminating system 300. The first photomask 200 has a first region 210 and a second region 220 corresponding to the cell region 110 and the peripheral circuit region 120 on wafer 100 respectively. The first region 210 has a first pattern 230 corresponding to the dense and fine patterns 130 to be formed on the cell region 110 on wafer 100. The second region 220 is provided with a photo-shielding layer 240 to cover an entire surface of the second region 220. The first illuminating system 300 is a dipole modified illuminating system suitable to form the dense and fine pattern 130 oriented in one direction in the cell region 110. When the primary exposure is performed using the first photomask 200 and the first illuminating system 300, the dense pattern 130 in the cell region 110 are exposed to light, while the isolated pattern 140 in the peripheral circuit region 120 is not exposed to the light.
Next, referring to FIGS. 4 and 5, a secondary exposure is performed using a second photomask 400 and a second illuminating system 500. The second photomask 400 has a first region 410 and a second region 420 corresponding to the cell region 110 and the peripheral circuit region 120 on wafer 100 respectively. The first region 410 is provided with a photo-shielding layer 430 to cover the entire surface of the first region 410. The second region 420 has a second pattern 440 corresponding to the isolated pattern 120 to be formed on the peripheral circuit region 120 of wafer 100. The second illuminating system 500 is an annular modified illuminating system, suitable to form the isolated pattern 140 having a higher pitch and disposed in more than one directions as in the peripheral circuit region 120. When the secondary exposure is performed using the second photomask 400 and the second illuminating system 500, the isolated pattern 140 of the peripheral circuit region 120 is exposed to light, whereas the dense and fine pattern 130 in the cell region 110 are not exposed to the light.
In the conventional double exposure method, the first photomask 200 and the first illuminating system 300 are used to form the pattern in the cell region 110, and the second photomask 400 and the second illuminating system 500 are used to form the pattern in the peripheral circuit region 120, thereby allowing the dense and fine patterns 130 and the isolated pattern 140 to be formed at an excellent resolution on the cell region 110 and the peripheral circuit region 120, respectively. However, since it is necessary to provide the first photomask 200 for the primary exposure and the second photomask 400 for the secondary exposure, manufacturing costs are increased. In addition, changing of the photomasks between primary and secondary exposures complicates the overall process of manufacturing semiconductor devices and increases manufacturing time.